KR20210008740A - Method for manufacturing positive electrode active material - Google Patents

Abstract

The present invention relates to a production method of a positive electrode active material, comprising the steps of: dry-mixing lithium composite transition metal oxide with polyimide precursor particles; and forming a polyimide coating layer having a glass transition temperature, Tg, of 300°C or less on the lithium composite transition metal oxide by heat-treating the mixture, wherein the heat treatment is performed at a temperature equal to or higher than the glass transition temperature, Tg, of the polyimide coating layer and, more specifically, to a production method of a positive electrode active material, capable of improving service life characteristics of the positive electrode active material.

Description

Manufacturing method of positive electrode active material{METHOD FOR MANUFACTURING POSITIVE ELECTRODE ACTIVE MATERIAL}

The present invention relates to a method of manufacturing a positive electrode active material.

In recent years, with the rapid spread of electronic devices using batteries such as mobile phones, notebook computers, and electric vehicles, the demand for small, lightweight, and relatively high-capacity secondary batteries is rapidly increasing. In particular, lithium secondary batteries are in the spotlight as a driving power source for portable devices because they are lightweight and have high energy density. Accordingly, research and development efforts for improving the performance of lithium secondary batteries are being actively conducted.

In general, a lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, an electrolyte, an organic solvent, and the like. In addition, in the positive electrode and the negative electrode, an active material layer including a positive active material or a negative active material may be formed on the current collector.

However, as the lithium secondary battery is repeatedly charged and discharged, there is a problem that its lifespan rapidly decreases. This deterioration in life characteristics is due to a side reaction between the anode and the electrolyte, and this phenomenon may become more serious under high voltage and high temperature conditions. Therefore, it is necessary to develop a secondary battery for high voltage, and for this purpose, a technology for controlling a side reaction between a positive electrode active material and an electrolyte or an electrode interface reaction is very important.

In order to solve this problem, a technique for reducing side reactions with the electrolyte has been proposed by coating a metal oxide such as Al ₂ O ₃ , ZrO ₂ , and AlPO ₄ on the surface of the positive electrode active material to suppress contact between the positive electrode active material and the electrolyte.

However, since the coating layer formed using the metal oxide is an ion insulating layer in which lithium ions are difficult to move, there is a problem in that the lithium ion conductivity is lowered.

Meanwhile, Korean Patent No. 1105342 discloses that a polymer solution is prepared by dissolving polyamic acid in a solvent, and then a positive electrode active material is dispersed in the polymer solution, and then the solvent is removed and heat treated to form a film-type polyimide on the surface of the positive electrode active material. A technique for forming a coating layer is disclosed.

When the polyimide coating layer is formed by the above method, there is an advantage in that the polyimide forms a thin coating layer, thereby preventing contact between the positive electrode active material and the electrolyte. However, when the coating layer in the form of a film is formed on the surface of the positive electrode active material, there is a problem in that the lithium ion conductivity decreases, the resistance increases, and the capacity decreases. In addition, the method uses a wet coating method using a solution, but there is also a problem in that fairness is inferior, such as layer separation, etc., occurs in the polymer solution used for wet coating, and requires a separate device and process for coating. have.

Korean Patent Registration No. 10-1105342

An object of the present invention is to form a polyimide coating layer having a specific range of glass transition temperature on a positive electrode active material by heat treatment at or above the glass transition temperature by using a dry mixing method, thereby providing a polyimide having excellent surface adhesion and coverage. It is to provide a method of manufacturing a positive electrode active material capable of forming a coating layer and improving the life characteristics of the positive electrode active material.

The present invention comprises the steps of dry mixing a lithium composite transition metal oxide and a polyimide precursor particle; And forming a polyimide coating layer having a glass transition temperature T _g of 300° C. or less on the lithium composite transition metal oxide by heat treatment, wherein the heat treatment is performed at a temperature of the glass transition temperature T _g or more of the polyimide coating layer. It provides a method of manufacturing a positive active material that is performed.

In the method for preparing a positive electrode active material of the present invention, in introducing the polyimide coating layer on the lithium composite transition metal oxide, the polyimide precursor particles and the lithium composite transition metal oxide are dry-mixed, and then the mixture is heat-treated. At this time, the glass transition temperature Tg of the polyimide coating layer is 300°C or less, and the heat treatment is performed at a temperature higher than the glass transition temperature Tg. Since the polyimide coating layer used in the present invention has a low level of glass transition temperature, it has flowability during heat treatment at a temperature higher than the glass transition temperature, and is capable of forming a polyimide coating layer having high coverage and high surface adhesion on the surface of the positive electrode active material. It is possible.

In addition, according to the manufacturing method of the positive electrode active material of the present invention, it is possible to prevent the possibility of contamination of the positive electrode active material due to the wet coating method, the increase in production cost and production time due to the introduction of an additional process, and to form a coating layer through the dry coating method. Therefore, a separate equipment for forming the coating layer is not required, the coating layer forming process is simple, and problems such as a decrease in initial discharge capacity and an increase in resistance of the cathode material that occur when applying wet coating can be minimized. Accordingly, the secondary battery to which the positive active material manufactured from the manufacturing method of the present invention is applied may have superior initial capacity characteristics, resistance characteristics, and life characteristics compared to the prior art.

The terms or words used in this specification and claims should not be construed as being limited to their usual or dictionary meanings, and the inventor may appropriately define the concept of terms in order to describe his own invention in the best way. It should be interpreted as a meaning and concept consistent with the technical idea of the present invention based on the principle that there is.

The terms used in the present specification are only used to describe exemplary embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise.

In the present specification, terms such as "comprise", "include" or "have" are intended to designate the presence of implemented features, numbers, steps, components, or a combination thereof, and one or more other features or It is to be understood that the possibility of the presence or addition of numbers, steps, elements, or combinations thereof is not preliminarily excluded.

In the present invention, the average particle diameter (D ₅₀ ) can be defined as a particle diameter based on 50% of a volume cumulative particle diameter distribution, and can be measured using a laser diffraction method. Specifically, the average particle diameter (D ₅₀ ) is according to the particle diameter measured from the scattered light and injected into a commercially available laser diffraction particle size measuring device (eg Horiba Partica LA-960) after dispersing the target particles in a dispersion medium. It is possible to calculate the average particle diameter (D ₅₀ ) based on 50% of the cumulative particle volume distribution.

Hereinafter, the present invention will be described in detail.

<Method of manufacturing positive electrode active material>

The present invention relates to a method of manufacturing a positive electrode active material, specifically, to a method of manufacturing a positive electrode active material for a lithium secondary battery.

The method for preparing a positive electrode active material of the present invention comprises the steps of dry mixing a lithium composite transition metal oxide and a polyimide precursor particle; And forming a polyimide coating layer having a glass transition temperature T _g of 300° C. or less on the lithium composite transition metal oxide by heat treatment, wherein the heat treatment is performed at a temperature of the glass transition temperature T _g or more of the polyimide coating layer. Performed.

According to the method for preparing a positive electrode active material of the present invention, in forming a coating layer on a lithium composite transition metal oxide, mixing of the coating layer forming precursor and the lithium composite transition metal oxide is not performed by a wet mixing method, and an organic solvent is not used. Do not do by dry mixing. Accordingly, the manufacturing method of the present invention can exclude the use of an organic solvent by a wet coating method, and prevent contamination of a lithium complex transition metal oxide or a positive electrode active material, side reactions, etc. due to direct contact between the lithium complex transition metal oxide and the organic solvent. It can be prevented, and since it is not necessary to introduce additional processes such as washing of the produced positive electrode active material and removal of an organic solvent, an increase in production efficiency can be expected.

In addition, according to the method of manufacturing a positive electrode active material of the present invention, a polyimide coating layer having a low level of glass transition temperature is formed, and accordingly, after dry mixing of the lithium composite transition metal oxide and the polyimide precursor particles, the glass When heat treatment at a temperature higher than the transition temperature, the flowability of the polyimide can be improved to an excellent level, so that the surface of the positive electrode active material can be coated at an excellent level, and a polyimide coating layer having high surface adhesion can be formed. Therefore, it is possible to solve the problems of low coverage and low surface adhesion of the coating layer of the conventional dry coating method or dry mixing method.

The manufacturing method of the positive electrode active material of the present invention includes the step of dry mixing a lithium composite transition metal oxide and a polyimide precursor particle.

The lithium composite transition metal oxide may be a material capable of reversible insertion/desorption of lithium, and specifically, the lithium composite transition metal oxide may include lithium (Li) and a transition metal.

The lithium composite transition metal oxide may further include at least one metal selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), and aluminum (Al).

In the lithium composite transition metal oxide, the positive electrode active material is lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), Li[Ni _x Co _y Mn _z Mv]O ₂ (wherein M is Al, Ga, and In Any one selected from the group consisting of or two or more of these elements; 0.3≤x<1.0, 0≤y, z≤0.5, 0≤v≤0.1, x+y+z+v=1), Li (Li _a M _ba-b' M'b _' ) O _2-c A _c (in the above formula, 0≤a≤0.2, 0.6≤b≤1, 0≤b'≤0.2, 0≤c≤0.2; M Contains Mn and at least one selected from the group consisting of Ni, Co, Fe, Cr, V, Cu, Zn and Ti; M'is at least one selected from the group consisting of Al, Mg and B , A is at least one selected from the group consisting of P, F, S and N), such as layered compounds or compounds substituted with one or more transition metals; Lithium manganese oxides such as the formula Li _1+y Mn _2-y O ₄ (wherein y is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , and LiMnO ₂ ; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ , and Cu ₂ V ₂ O ₇ ; Ni site-type lithium nickel oxide represented by the formula LiNi _1-y M _y O ₂ (here, M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, and y is 0.01 to 0.3); Formula LiMn _2-y M _y O ₂ (where M = Co, Ni, Fe, Cr, Zn or Ta, y is 0.01 to 0.1) or Li ₂ Mn ₃ MO ₈ (where M = Fe, Co, A lithium manganese composite oxide represented by Ni, Cu, or Zn); LiMn ₂ O _{4 in} which part of Li in the formula is substituted with alkaline earth metal ions; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ and the like may be mentioned, but the present invention is not limited thereto.

Preferably, the positive electrode active material may include a lithium composite transition metal oxide represented by Formula a below.

[Formula a]

Li _x [Ni _y Co _z Mn _w M ¹ _v ]O _2-p A _p

In the [Formula a], the M ¹ is a doping element substituted on the transition metal site, W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, Ta, Y, In, La, Sr, Ga , Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and may be one or more elements selected from the group consisting of Mo.

The A is an element substituted at the oxygen site, and may be at least one element selected from the group consisting of F, Cl, Br, I, At, and S.

The x refers to the atomic ratio of lithium to the total transition metal in the lithium composite transition metal oxide, and may be 1 to 1.30. When the atomic ratio of lithium satisfies the above range, it is possible to obtain a lithium composite transition metal oxide having a high crystallinity and little cation mixing while the average particle diameter of the primary particles is 3 μm or more.

Wherein y refers to the atomic ratio of nickel in the transition metal in the lithium composite transition metal oxide, 0<y<1, preferably 0.2≤y≤0.95, more preferably 0.5≤y≤0.95, the range If present, it is preferable in terms of implementing a high capacity of the active material.

Wherein z refers to the atomic ratio of cobalt in the transition metal in the lithium composite transition metal oxide, 0<y<1, preferably 0<y≤0.6, preferably 0.01≤y≤0.4.

The w refers to the ratio of manganese atoms in the transition metal in the lithium composite transition metal oxide, and is 0<y<1, preferably 0<y≤0.6, preferably 0.01≤y≤0.4.

The v denotes the atomic ratio of the doped element M ¹ doped to the transition metal site in the lithium composite transition metal oxide, and may be 0≦v≦0.2, preferably 0≦v≦0.1. When the doping element M ¹ is added, there is an effect of improving the structural stability of the lithium composite transition metal oxide. However, since the capacity may decrease when the content of the doping element increases, it is preferably contained in an amount of 0.2 or less.

The p denotes the atomic ratio of the element A substituted in the oxygen site, and may be 0≦p≦0.2, preferably 0≦p≦0.1.

Meanwhile, in Chemical Formula a, y+z+w+v=1.

The average particle diameter (D ₅₀ ) of the lithium composite transition metal oxide may be 4 μm to 25 μm, more preferably 8 μm to 18 μm, and when in the above-described range, the surface area of the lithium composite transition metal oxide Since it is implemented at an appropriate level, the polyimide coating layer can be uniformly formed without agglomeration of the polyimide.

The polyimide precursor particles are a component that becomes a precursor of the polyimide coating layer, and a polyimide coating layer is formed on the surface of the lithium composite transition metal oxide by heat treatment to be described later.

The polyimide precursor particles may be cured by heat treatment to form a polyimide coating layer having a glass transition temperature T _g of 300°C or less. The implementation of the polyimide coating layer having the glass transition temperature T _g range may be performed by controlling the chemical structure and organic groups in the polyimide precursor particles and/or the polyimide coating layer.

Specifically, the polyimide precursor particles may include polyamic acid and/or polyamic acid ester, and specifically, may include a repeating unit represented by Formula 1 below.

[Formula 1]

In the repeating unit of Formula 1, A is a tetravalent organic group having 4 to 30 carbon atoms, B is a divalent organic group having 6 to 30 carbon atoms, and at least one of A and B is -O- and -COO- It includes at least one functional group selected from the group consisting of, R ₁ and R ₂ are each independently H or an alkyl group having 1 to 5 carbon atoms, and n is an integer of 1 or more.

Of the repeating units of Formula 1, at least one of A and B contains one or more functional groups selected from the group consisting of -O- and -COO-, and the polyimide precursor particles including the repeating unit are at a low level by heat treatment. It is possible to form a polyimide coating layer having a glass transition temperature T _{g of} , and does not penetrate into the lithium composite transition metal oxide particles during the curing process, and is used for surface coating, and an increase in resistance of the active material can be minimized.

Specifically, A may be selected from a tetravalent organic group represented by Formula 2a, Formula 2b, Formula 2c, and Formula 2d below.

[Formula 2a]

[Formula 2b]

[Formula 2c]

[Formula 2d]

In Formula 2a, Formula 2b, Formula 2c, and Formula 2d, * indicates a binding site.

The B may be selected from a divalent organic group represented by the following Formula 3a, Formula 3b, Formula 3c, Formula 3d, Formula 3e, Formula 3f, and Formula 3g.

[Formula 3a]

[Formula 3b]

[Formula 3c]

[Formula 3d]

[Formula 3e]

[Chemical Formula 3f]

[Chemical Formula 3g]

In Formula 3a, Formula 3b, Formula 3c, Formula 3d, Formula 3e, Formula 3f, and Formula 3g, * denotes a binding site.

In the repeating unit represented by Formula 1, wherein A is selected from a tetravalent organic group represented by Formula 2a, Formula 2b, Formula 2c and Formula 2d, or B is Formula 3a, Formula 3b, Formula 3c , Formula 3d, Formula 3e, Formula 3f, and Formula 3g may be selected from the divalent organic group represented by the. Specifically, A is selected from a tetravalent organic group represented by Chemical Formula 2a, Chemical Formula 2b, Chemical Formula 2c, and Chemical Formula 2d, and B is 2 which does not contain a functional group selected from the group consisting of -O- and -COO- It may be selected from among the organic groups. Or, A is a tetravalent organic group that does not contain a functional group selected from the group consisting of -O- and -COO-, and B is the formula 3a, formula 3b, formula 3c, formula 3d, formula 3e, formula 3f, and formula It may be a divalent organic group represented by 3g. Or wherein A is selected from a tetravalent organic group represented by Formula 2a, Formula 2b, Formula 2c and Formula 2d, and B is Formula 3a, Formula 3b, Formula 3c, Formula 3d, Formula 3e, Formula 3f, and Formula It may be one selected from divalent organic groups represented by 3g.

The polyimide precursor particles have a particle shape.

The polyamide precursor particles may be prepared through a collision mixing process.

The collision mixing process is a process of rapidly mixing and spraying two solvents having different solubility in a solute to produce a polymer nanopowder. More specifically, a solution containing a solute and a good solvent having a high solubility in the solute (for convenience This is a method of forming particles by spraying a solution containing a first solution) and a non-solvent in which the solute is not dissolved (referred to as a second solution for convenience) from different directions at high speed to collide with each other.

In the present invention, the solute used in the collision mixing process may be polyamic acid and/or polyamic acid ester, and the polyamic acid and/or polyamic acid ester is 0.1 wt% to 40 wt%, preferably in the first solution May be included in a concentration of 0.5% to 30% by weight. When the concentration of the polyamic acid and/or the polyamic acid ester satisfies the above range, high-speed spraying and particle formation may be smoothly performed.

Conventionally, a precipitation method and a spray method have been mainly used as technologies for producing polyimide precursor particles. In the precipitation method, a non-solvent is added to the reactor and stirred, and a polymer solution in which polyamic acid and/or polyamic acid ester is dissolved in a good solvent is added dropwise to the non-solvent, thereby replacing the solvent between the non-solvent and the good solvent. It is a method of forming mid precursor particles. The method of manufacturing polyimide precursor particles by the precipitation method has the advantage that the process is simple and economical, but there is a problem that the particle size is formed in a micro size, and aggregation occurs during particle production, making it difficult to control the particle shape and particle size distribution. On the other hand, the spray method is a method of forming particles by heating and drying a polymer solution containing polyamic acid at the same time as spraying, and has the advantage that the amount of solvent is small and small particles can be formed compared to the precipitation method, but the particle size is reduced to nano size. Is difficult, and a heating process must be accompanied, there is a problem that there is a risk of explosion of the organic solvent, and the polyamic acid may be deteriorated by heating.

In contrast, in the case of the collision mixing process, when the first solution and the second solution are mixed, solvent exchange occurs between the good solvent and the non-solvent, and the polyimide precursor particles are formed as the nucleus grows. Since the mixing is carried out in a collision mixing method through spraying and the reaction volume is small, nano-sized small particles are formed, and the mixing efficiency can be increased, so that the particle size also has a relatively uniform distribution. In addition, in the case of the collision mixing process, since the solvent is removed without a separate heating process, the polymer powder can be prepared without chemical or physical alteration.

The good solvent is a solvent having high solubility in polyamic acid and/or polyamic acid ester, and an appropriate solvent may be selected and used according to the type of polymer or polymer precursor used. For example, the good solvent may contain at least one selected from the group consisting of N-methyl pyrrolidone, dimethylformamide, dimethyl sulfoxide, tetrahydrofuran, acetone, ethyl acetate, acetonitrile, and acetic acid. .

The non-solvent refers to a solvent in which the polyamic acid and/or polyamic acid ester is not dissolved or has an extremely low solubility in the polyamic acid and/or polyamic acid ester, for example, water, isopropyl alcohol, methanol, ethanol , Alcohol-based solvents such as butanol, hydrocarbon-based solvents such as hexane and cyclohexane, and at least one selected from the group consisting of benzene-based solvents such as toluene and xylene.

The collision mixing process as described above may be performed using, for example, an impingement mixer such as a T-jet mixer.

The average particle diameter (D ₅₀ ) of the polyimide precursor particles may be 1,000 nm or less, preferably 1 nm to 800 nm, more preferably 5 nm to 500 nm, even more preferably 100 nm to 300 nm, and the average particle diameter of the range ( When using the polyimide precursor particles having D ₅₀ ), the coating material may be uniformly distributed in the positive electrode active material even when the coating layer is formed through dry mixing.

In addition, the polyimide precursor particles may have a uniform size, and a span value may be 2.0 or less, specifically 0.5 to 2.0, and more specifically 0.8 to 1.5.

In this way, when polyimide precursor particles having a small particle size and high particle size uniformity are used, a uniform coating layer can be formed even when a dry coating method is used, unlike the case of using the polyimide precursor particles of the micro-size level. Accordingly, a positive electrode active material having excellent resistance characteristics and capacity characteristics can be prepared.

The lithium composite transition metal oxide and the polyimide precursor particles are dry mixed. The dry mixing refers to mixing without using a solvent, and is a concept distinguished from wet mixing using a solvent during mixing.

According to the method of manufacturing the positive electrode active material of the present invention, since the lithium composite transition metal oxide and the polyimide precursor particles are dry mixed, contamination of the positive electrode active material due to direct contact of the organic solvent with the lithium composite transition metal oxide, side reactions, Due to this, the problems of initial capacity drop and resistance increase can be effectively prevented.

In addition, according to the present invention, since it is not necessary to introduce additional processes for washing the positive electrode active material, removing the organic solvent, etc., which are problematic during wet coating, the production time of the positive electrode active material is reduced, and production cost can be reduced.

In addition, as a problem of conventional dry mixing or dry coating, low coverage and low surface adhesion of the coating layer components are a problem. However, as described above, the present invention uses polyimide precursor particles of 300° C. or lower as a coating layer forming precursor. It is possible to form a coating layer having coverage and high surface adhesion.

The dry mixing refers to mixing without using a solvent, and is well known in the art such as ball-mill, jet-mill, pin mill, and sieving, stirring mixing, impact mixing, and acoustic mixing. It can be carried out using a mixing method.

The polyimide precursor particles may be dry mixed in an amount of 0.01 to 1 parts by weight, preferably 0.05 to 0.5 parts by weight, based on 100 parts by weight of the lithium composite transition metal oxide. When the lithium composite transition metal oxide particles and the polyimide precursor particles are mixed in the above amount, the polyimide coating layer can sufficiently coat the surface of the lithium composite transition metal oxide, so that the life and stability of the positive electrode active material can be improved. Problems of a decrease in capacity and an increase in resistance due to excessive formation of the mid coating layer can be prevented.

The method for preparing a positive electrode active material of the present invention includes the step of forming a polyimide coating layer having a glass transition temperature T _g of 300° C. or less on the lithium composite transition metal oxide by heat-treating the mixture after dry mixing, and It is carried out at a temperature _{equal to} or higher than the glass transition temperature T _g of the polyimide coating layer.

According to the present invention, by performing the heat treatment process after dry mixing, the polyimide precursor particles are converted to polyimide by imidization to form a polyimide coating layer having a glass transition temperature T _g of 300°C or less. Since the heat treatment is performed at a temperature equal to or higher than the glass transition temperature of the polyimide coating layer, the polyimide precursor particles or the polyimide formed therefrom form a polyimide coating layer having excellent flow properties on the surface of the lithium composite transition metal oxide particles. Is done.

The polyimide formed by converting the polyimide precursor particles may be an amorphous polymer rather than a crystalline polymer, and does not have a melting temperature (T _m ) and a glass transition temperature (T _g) It may be a material having only ). Accordingly, the polyimide does not penetrate into the inside of the lithium composite transition metal oxide during the curing process and can be used for surface coating, thereby minimizing an increase in resistance of the active material.

The polyimide coating layer has a glass transition temperature T _g of 300°C or less. Typically, the polyimide coating layer has a glass transition temperature T _g of more than 300°C, and high flowability cannot be expected during heat treatment after dry mixing, and thus, it sufficiently covers the lithium composite transition metal oxide surface during dry mixing or dry coating. Or can't cover. In addition, when heat treatment is performed at a higher temperature in order to improve the flowability of the polyimide having a glass transition temperature of more than 300°C, it is not preferable because there is a risk of thermal decomposition of the polymer. Therefore, when a coating layer is formed on a lithium composite transition metal oxide with a conventional polyimide, it is difficult to expect an improvement in the stability of the positive electrode active material, and a side reaction with an electrolyte solution, resulting in a decrease in lifespan characteristics, cannot be prevented.

However, since the polyimide coating layer of the present invention has a low level of glass transition temperature, it may have high flowability, high surface adhesion, and high coverage on the surface of the lithium composite transition metal oxide particles during heat treatment after dry mixing. Therefore, the positive electrode active material manufactured by the manufacturing method of the present invention is preferable because it can have high stability and high lifespan characteristics.

The polyimide coating layer may have a glass transition temperature T _g of 300°C or less, preferably 230°C to 300°C, and more preferably 235°C to 280°C. The polyimide coating layer having a glass transition temperature in the above range can be expected to have higher flowability during heat treatment, and a polyimide coating layer having high surface adhesion and coverage can be formed.

The glass transition temperature of the polyimide coating layer may be determined by a method of controlling the chemical structure and organic groups of the polyimide precursor particles, and is implemented by using, for example, polyimide precursor particles containing the repeating unit of Formula 1 above. Can be.

The glass transition temperature of the polyimide coating layer may be measured by Thermal Mechanical Analysis (TMA) or Differential Scanning Calorimetry (DSC).

The polyimide coating layer may include a repeating unit of Formula 4 below.

[Formula 4]

In Formula 4, A and B are the same as A and B in Formula 1, and m is an integer of 1 or more.

The heat treatment is performed at a temperature equal to or higher than the glass transition temperature of the coating layer in order to improve the flowability of the polyimide coating layer. Specifically, the heat treatment temperature may be 250° C. to 400° C., preferably 260° C. to 310° C., and when heat treatment is performed at a temperature within the above range, the flowability of polyimide is improved so that a coating layer having high coverage can be formed, While preventing the polyimide from thermally decomposing, it is preferable from the viewpoint of improving heat resistance by imidization of the polyimide precursor particles.

The heat treatment is 0.1 to 6 hours in terms of forming a polyimide coating layer having sufficient coverage on the lithium composite transition metal oxide, and preventing the problem of dissolving the polyimide coating layer in a solvent or the like due to insufficient imidation and falling off the surface. , Preferably it can be carried out for 0.5 to 3 hours.

<Anode>

The present invention provides a positive electrode including the positive electrode active material manufactured by the above-described manufacturing method. More specifically, the positive electrode may be a positive electrode for a lithium secondary battery.

Specifically, the positive electrode is a positive electrode current collector; A positive electrode active material layer formed on the positive electrode current collector may be included, and the positive electrode active material layer may include the aforementioned positive electrode active material.

In the positive electrode, the positive electrode current collector is not particularly limited as long as it has conductivity without causing chemical changes to the battery, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or carbon on the surface of aluminum or stainless steel. , Nickel, titanium, silver, or the like may be used. In addition, the positive electrode current collector may generally have a thickness of 3 to 500 μm, and fine unevenness may be formed on the surface of the positive electrode current collector to increase the adhesion of the positive electrode active material or the positive electrode material. For example, it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

The positive electrode active material layer may include a conductive material and a binder in addition to the positive electrode active material described above.

The positive active material may be included in an amount of 80 to 99% by weight in the positive active material layer.

The conductive material is used to impart conductivity to the electrode, and in the battery to be configured, it can be used without particular limitation as long as it does not cause chemical changes and has electronic conductivity. Specific examples include graphite such as natural graphite and artificial graphite; Carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; Metal powder or metal fibers such as copper, nickel, aluminum, and silver; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Alternatively, a conductive polymer such as a polyphenylene derivative may be used, and one of them alone or a mixture of two or more may be used. The conductive material may be included in an amount of 1 to 30% by weight in the positive active material layer.

The binder serves to improve adhesion between the positive electrode active material and adhesion between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC). ), starch, hydroxypropylcellulose, recycled cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and the like, and one of them alone or a mixture of two or more may be used. The binder may be included in an amount of 1 to 30% by weight in the positive active material layer.

The positive electrode may be manufactured according to a conventional positive electrode manufacturing method except for using the positive electrode active material described above. Specifically, the composition for forming a positive electrode active material layer including the positive electrode active material, optionally a binder and a conductive material may be coated on a positive electrode current collector, followed by drying and rolling.

The solvent may be a solvent generally used in the art, dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or Water, and the like, and one of them alone or a mixture of two or more may be used. The amount of the solvent is sufficient to dissolve or disperse the positive electrode active material, the conductive material, and the binder in consideration of the coating thickness and production yield of the slurry, and then have a viscosity that can exhibit excellent thickness uniformity when applied for the production of the positive electrode. Do.

Alternatively, the positive electrode may be prepared by casting the composition for forming the positive electrode active material layer on a separate support, and then laminating a film obtained by peeling from the support on a positive electrode current collector.

Secondary battery

In addition, the present invention provides an electrochemical device including the anode. The electrochemical device may specifically be a secondary battery or a capacitor, and more specifically, a lithium secondary battery.

The secondary battery specifically includes a positive electrode, a negative electrode positioned opposite the positive electrode, a separator and an electrolyte interposed between the positive electrode and the negative electrode, and the positive electrode is the same as the above-described positive electrode. In addition, the secondary battery may optionally further include a battery container accommodating the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

In the secondary battery, the negative electrode includes a negative electrode current collector and a negative active material layer disposed on the negative electrode current collector.

The negative electrode current collector is not particularly limited as long as it has high conductivity without causing a chemical change in the battery, for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel. Surface treatment with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, and the like may be used. In addition, the negative electrode current collector may generally have a thickness of 3 to 500 μm, and, like the positive electrode current collector, microscopic irregularities may be formed on the surface of the current collector to enhance the bonding strength of the negative active material. For example, it may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

The negative active material layer optionally includes a binder and a conductive material together with the negative active material. The negative electrode active material layer is, for example, coated on a negative electrode current collector and a composition for forming a negative electrode including a binder and a conductive material is applied and dried, or the composition for forming the negative electrode is cast on a separate support. , It can also be produced by laminating a film obtained by peeling from this support on a negative electrode current collector.

As the negative active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; Metal compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; Metal oxides capable of doping and undoping lithium such as SiO _β (0 <β <2), SnO ₂ , vanadium oxide, and lithium vanadium oxide; Or a composite including the metal compound and a carbonaceous material, such as a Si-C composite or a Sn-C composite, and any one or a mixture of two or more of them may be used. In addition, a metal lithium thin film may be used as the negative electrode active material. Further, as the carbon material, both low crystalline carbon and high crystalline carbon may be used. As low crystalline carbon, soft carbon and hard carbon are typical, and high crystalline carbon is amorphous, plate-like, scale-like, spherical or fibrous natural or artificial graphite, Kish graphite (Kish). graphite), pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches, and petroleum or coal tar pitch High-temperature calcined carbon such as derived cokes) is representative.

In addition, the binder and the conductive material may be the same as described above for the positive electrode.

Meanwhile, in the secondary battery, the separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move, and can be used without particular limitation as long as it is generally used as a separator in a secondary battery. It is preferable that it is resistance and has excellent electrolyte-moisturizing ability. Specifically, a porous polymer film, for example, a porous polymer film made of polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer, and ethylene/methacrylate copolymer, or these A stacked structure of two or more layers of may be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of a high melting point glass fiber, polyethylene terephthalate fiber, or the like may be used. In addition, in order to secure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material may be used, and optionally, a single layer or a multilayer structure may be used.

In addition, the electrolytes used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc. that can be used in the manufacture of secondary batteries. It is not.

Specifically, the electrolyte may include an organic solvent and a lithium salt.

The organic solvent may be used without particular limitation as long as it can serve as a medium through which ions involved in the electrochemical reaction of a battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; Ether solvents such as dibutyl ether or tetrahydrofuran; Ketone solvents such as cyclohexanone; Aromatic hydrocarbon solvents such as benzene and fluorobenzene; Dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), propylene carbonate, Carbonate-based solvents such as PC); Alcohol solvents such as ethyl alcohol and isopropyl alcohol; Nitriles such as R-CN (R is a C2 to C20 linear, branched or cyclic hydrocarbon group and may contain a double bonded aromatic ring or an ether bond); Amides such as dimethylformamide; Dioxolanes such as 1,3-dioxolane; Alternatively, sulfolanes or the like may be used. Among them, carbonate-based solvents are preferable, and cyclic carbonates having high ionic conductivity and high dielectric constant (e.g., ethylene carbonate or propylene carbonate, etc.), which can increase the charging/discharging performance of the battery, and low-viscosity linear carbonate-based compounds ( For example, a mixture of ethylmethyl carbonate, dimethyl carbonate or diethyl carbonate) is more preferable. In this case, when the cyclic carbonate and the chain carbonate are mixed and used in a volume ratio of about 1:1 to about 1:9, the electrolyte may exhibit excellent performance.

The lithium salt may be used without particular limitation as long as it is a compound capable of providing lithium ions used in secondary batteries. Specifically, the lithium salt is LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAl0 ₄ , LiAlCl ₄ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN(C ₂ F ₅ SO ₃ ) ₂ , LiN(C ₂ F ₅ SO ₂ ) ₂ , LiN(CF ₃ SO ₂ ) ₂ . LiCl, LiI, or LiB(C ₂ O ₄ ) ₂ or the like may be used. The concentration of the lithium salt is preferably used within the range of 0.1 to 2.0M. When the concentration of the lithium salt is within the above range, since the electrolyte has an appropriate conductivity and viscosity, excellent electrolyte performance can be exhibited, and lithium ions can effectively move.

In addition to the electrolyte constituents, the electrolyte includes, for example, haloalkylene carbonate-based compounds such as difluoroethylene carbonate, pyridine, and trivalent for the purpose of improving battery life characteristics, suppressing reduction in battery capacity, and improving battery discharge capacity. Ethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imida One or more additives, such as zolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol, or aluminum trichloride, may be further included. At this time, the additive may be included in an amount of 0.1 to 5% by weight based on the total weight of the electrolyte.

The secondary battery using the positive electrode active material manufactured by the manufacturing method of the present invention as described above can stably exhibit excellent life characteristics, portable devices such as mobile phones, notebook computers, digital cameras, and hybrid electric vehicles. vehicle, HEV) and other electric vehicles.

Accordingly, the present invention provides a battery module including the secondary battery as a unit cell and a battery pack including the same.

The battery module or battery pack may include a power tool; Electric vehicles including electric vehicles (EV), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEV); Alternatively, it may be used as a power source for any one or more medium and large-sized devices in a power storage system.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein.

Manufacturing example

Preparation Example 1: Preparation of polyimide precursor particles

2,2'-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) 13 g was mixed with N-methyl-2-pyrrolidone (NMP) and tetrahydrofuran in a weight ratio of 3:97 It was dissolved in 180.4 g of solvent. Thereafter, 14 g of 4,4'-(hexafluoroisopropylidene) bis-phthalic anhydride (6FDA) was added to the solution under an ice bath, and then stirred at room temperature for 116 hours to form the first solution (polyimide precursor solution). ) Was prepared. Distilled water was prepared as the second solution (non-solvent).

Using a T-jet mixer, the first solution and the second solution were sprayed at high speed at a spray rate of 50 ml/min, respectively, to perform a collision mixing process. The mixed solution obtained by collision mixing of the first solution and the second solution was stirred in 10 times the volume of distilled water, and then only particles were obtained through a centrifuge, and then dried to prepare the polyimide precursor particles of Preparation Example 1 ( Average particle diameter (D ₅₀ ): 190 nm, dispersion degree: 0.93).

The average particle diameter and dispersion degree of the polyimide precursor particles were measured using a laser diffraction particle size measuring apparatus (Horiba Partica LA-960).

Preparation Example 2: Preparation of polyimide precursor particles

2,2'-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) 20 g was mixed with N-methyl-2-pyrrolidone (NMP) and tetrahydrofuran in a weight ratio of 3:97 It was dissolved in 136.5 g of solvent. In addition, 14 g of 4,4'-biphthalic anhydride (4,4'-biphthalic anhydride) (BPDA) was added to the solution under an ice bath, and then stirred at room temperature for 16 hours to form the first solution ( Polyimide precursor solution) was prepared.

Polyimide precursor particles were prepared in the same manner as in Example 1, except that the first solution prepared above was used (average particle diameter (D ₅₀ ): 205 nm, dispersion degree: 1.05).

Preparation Example 3: Preparation of polyimide precursor particles

11 g of 2,2'-bis(trifluoromethyl)benzidine (TFMB) was dissolved in 107 g of a solvent in which N-methyl-2-pyrrolidone (NMP) and tetrahydrofuran were mixed in a weight ratio of 3:97. Then, 25 g of 2,2'-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA) was added to the solution under an ice bath, and then stirred at room temperature for 16 hours. A polyimide precursor solution was prepared.

Polyimide precursor particles were prepared in the same manner as in Example 1, except that the first solution prepared above was used (average particle diameter (D ₅₀ ): 207 nm, dispersion degree: 1).

Preparation Example 4: Preparation of polyimide precursor particles

13 g of 2,2'-bis(trifluoromethyl)benzidine (TFMB) was dissolved in 113 g of a solvent in which N-methyl-2-pyrrolidone (NMP) and tetrahydrofuran were mixed in a weight ratio of 3:97. In addition, 25 g of 4,4′-(hexafluoroisopropylidene) bis-phthalic anhydride (6FDA) was added to the solution under an ice bath, and then stirred at room temperature for 16 hours to prepare a polyimide precursor solution.

Polyimide precursor particles were prepared in the same manner as in Example 1, except that the first solution prepared above was used (average particle diameter (D ₅₀ ): 204 nm, dispersion degree: 1.05).

Example

Example 1: Preparation of positive electrode active material

<Dry Mix>

As a lithium composite transition metal oxide, LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (average particle diameter (D ₅₀ : 14 μm) and the polyimide precursor particles of Preparation Example 1 were dry-mixed at a weight ratio of 100:0.1.

<Heat treatment>

The mixture after dry mixing was heat-treated at 300° C. for 1 hour to prepare a positive electrode active material of Example 1 in which a polyimide coating layer was formed on a lithium composite transition metal oxide.

Example 2: Preparation of positive electrode active material

A positive electrode active material was prepared in the same manner as in Example 1, except that the polyimide precursor particles of Preparation Example 2 were used instead of the polyimide precursor particles of Preparation Example 1.

Example 3: Preparation of positive electrode active material

A positive electrode active material was prepared in the same manner as in Example 1, except that the polyimide precursor particles of Preparation Example 3 were used instead of the polyimide precursor particles of Preparation Example 1.

Comparative Example 1: Preparation of positive electrode active material

A positive electrode active material was prepared in the same manner as in Example 1, except that the polyimide precursor particles of Preparation Example 4 were used instead of the polyimide precursor particles of Preparation Example 1.

Comparative Example 2: Preparation of positive electrode active material

A positive electrode active material was prepared in the same manner as in Example 1, except that the polyimide precursor particles of Preparation Example 4 were used instead of the polyimide precursor particles of Preparation Example 1, and the heat treatment temperature was set to 350°C.

Comparative Example 3: Preparation of positive electrode active material

A positive electrode active material was prepared in the same manner as in Example 1, except that the heat treatment temperature was set to 230°C.

Comparative Example 4: Preparation of positive electrode active material

To 0.4 g of the first solution (polyimide precursor solution) prepared in Preparation Example 1, 39.6 g of N-methyl-2-pyrrolidone (NMP) was added and diluted.

LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (average particle diameter (D ₅₀ : 14 μm) 20 g) was added to the solution as a lithium composite transition metal oxide, and wet mixed for 1 hour.

After the mixed solution was filtered under reduced pressure, the active material coated with the polyimide precursor was heat-treated at 300° C., thereby preparing a positive electrode active material of Comparative Example 4 in which the polyimide coating layer was formed on a lithium composite transition metal oxide by a wet coating method.

Polyimide precursor Mixing method Polyimide coating layer Heat treatment temperature (℃) Glass transition temperature
(T _g , ℃) Example 1 Polyimide precursor particles of Preparation Example 1
deflation 259 300 Example 2 Polyimide precursor particles of Preparation Example 2 deflation 248 300 Example 3 Polyimide precursor particles of Preparation Example 3 deflation 235 300 Comparative Example 1 Polyimide precursor particles of Preparation Example 4 deflation 340 300 Comparative Example 2 Polyimide precursor particles of Preparation Example 4 deflation 340 350 Comparative Example 3 Polyimide precursor particles of Preparation Example 1 deflation 259 230 Comparative Example 4 Use of polyimide precursor solution in Preparation Example 1 (not in particle form) Wet 259 300

Experimental example

Experimental Example 1: Life evaluation

<Manufacture of secondary battery>

Examples 1 to 3, the positive electrode active material prepared in Comparative Examples 1 to 4, PVdF (KR9700, manufactured by KUREHA) as a binder, and carbon black (FX35, manufactured by DENKA) as a conductive material in an N-methylpyrrolidone solvent by weight A positive electrode slurry was prepared by mixing at a weight ratio of 97.5:1.0:1.5. This was applied to one side of an aluminum current collector (manufactured by Sanya Aluminum Co., Ltd.), dried at 80°C, and rolled to prepare a positive electrode.

Lithium metal was used as the negative electrode.

An electrode assembly was manufactured by interposing a porous polyethylene separator between the positive electrode and the negative electrode prepared as described above, and after placing the electrode assembly in the case, an electrolyte was injected into the case to manufacture a secondary battery.

<Evaluation of capacity retention rate>

Each of the secondary batteries of Examples and Comparative Examples prepared above was charged at 25°C with a constant current (CC) of 0.1 C until it reached 4.25 V, and then charged with a constant voltage (CV) so that the charging current was reduced to 0.05 C (cut -off current), the first charging was performed. After leaving it for 20 minutes, it was discharged until it became 2.5 V with a constant current (CC) of 0.1 C. Thereafter, charging and discharging was repeated at 0.1 C from 45° C. to 50 cycles to calculate the capacity of the last cycle compared to the first cycle, and the capacity retention rate was evaluated and shown in Table 2 below.

Experimental Example 2: Evaluation of discharge capacity and initial resistance

The secondary batteries of Examples and Comparative Examples prepared in Experimental Example 1 were charged at 25° C. with a constant current (CC) of 0.2 C until 4.25 V, and then charged with a constant voltage (CV) so that the charging current was 0.05 C (cut -off current), the first charging was performed. After leaving for 20 minutes, the discharge capacity was measured by discharging until 2.5 V with a constant current of 0.2 C (CC).

In addition, the initial resistance was measured by dividing the change in voltage from the full charge state to the initial discharge 60 seconds by the current.

The measurement results of the discharge capacity and initial resistance are shown in Table 2 below.

division Experimental Example 1 Experimental Example 2 Capacity retention rate (%) @ 50cycles Discharge capacity (mAh/g) Initial resistance (Ω) Example 1 94 204 28 Example 2 94 201 29 Example 3 94 205 31 Comparative Example 1 90 205 27 Comparative Example 2 89 198 64 Comparative Example 3 92 196 28 Comparative Example 4 94 188 47

Referring to Table 2, Examples 1 to 3 in which lithium composite transition metal oxide and polyimide precursor particles were dry mixed, the glass transition temperature of the polyimide coating layer was adjusted to 300°C or less, and heat-treated at a temperature equal to or higher than the glass transition temperature. In the case of the secondary battery of, it can be seen that compared to the secondary battery of the comparative examples, it exhibits excellent performance in terms of life characteristics, discharge capacity, and initial resistance.

Claims (10)

Dry-mixing the lithium composite transition metal oxide and the polyimide precursor particles; And
Heat-treating the mixture to form a polyimide coating layer having a glass transition temperature T _g of 300°C or less on the lithium composite transition metal oxide,
The heat treatment is performed at a temperature _{equal to} or higher than the glass transition temperature T _g of the polyimide coating layer.
The method according to claim 1,
The glass transition temperature T _g of the polyimide coating layer is 230°C to 300°C.
The method according to claim 1,
The polyimide precursor particle is a method for producing a positive electrode active material including a repeating unit represented by the following formula (1):
[Formula 1]

In Formula 1, A is a tetravalent organic group having 4 to 30 carbon atoms, B is a divalent organic group having 6 to 30 carbon atoms, and at least one of A and B is in the group consisting of -O- and -COO- It contains at least one selected functional group, R ₁ and R ₂ are each independently H or an alkyl group having 1 to 5 carbon atoms, and n is an integer of 1 or more.
The method of claim 3, wherein A is selected from tetravalent organic groups represented by the following Formulas 2a, 2b, 2c, and 2d:
[Formula 2a]

[Formula 2b]

[Formula 2c]

[Formula 2d]

In Formula 2a, Formula 2b, Formula 2c, and Formula 2d, * indicates a binding site.
The method of claim 3, wherein B is selected from a divalent organic group represented by the following Formula 3a, Formula 3b, Formula 3c, Formula 3d, Formula 3e, Formula 3f, and Formula 3g.
[Formula 3a]

[Formula 3b]

[Formula 3c]

[Formula 3d]

[Formula 3e]

[Chemical Formula 3f]

[Chemical Formula 3g]

In Formula 3a, Formula 3b, Formula 3c, Formula 3d, Formula 3e, Formula 3f, and Formula 3g, * denotes a binding site.
The method according to claim 1,
The average particle diameter (D ₅₀ ) of the polyimide precursor particles is 1,000 nm or less.
The method according to claim 1,
A method of manufacturing a positive electrode active material having a dispersion degree of the polyimide precursor particles of 2.0 or less.
The method according to claim 1,
The polyimide precursor particles are dry mixed in an amount of 0.01 to 1 part by weight based on 100 parts by weight of the lithium composite transition metal oxide.
The method according to claim 1,
The heat treatment is a method of manufacturing a positive active material performed at 250 ℃ to 400 ℃.
The method according to claim 1,
The heat treatment is a method of manufacturing a positive active material performed for 0.1 to 6 hours.